Posttraumatic stress disorder (PTSD) is a chronic and disabling condition that affects 6.5% of the U.S. population, or approximately 21 million Americans. The Institute of Medicine identified exposure therapy as the front-line PTSD treatment, yet 40% of individuals still meet PTSD criteria following exposure therapy, indicating a need for more effective treatments. To date, PTSD treatment research has been constrained by the lack of objective measures of the theorized mechanism of action: extinction of conditioned fear to trauma cues. New assessment tools are needed to measure this treatment target. Conditioned fear to trauma cues is typically measured by self-report, but self-report and physiological markers of fear responding differentially predict treatment outcome. Skin conductance (SC) is a commonly used physiological marker of fear, but SC is not a reliable marker for a substantial minority of individuals; there is a need for new objective markers. Two cardiovascular measures are promising: a specific marker of sympathetic arousal (pre-ejection period, PEP), and a marker of parasympathetic withdrawal (respiratory sinus arrhythmia, RSA). Current physiological assessments measure fear responses to a limited number of trauma cues presented in a controlled laboratory setting. These lab-based assessments differ in important ways from trauma reminders experienced in the ?real world,? limiting generalizability and clinical relevance. Ambulatory physiological assessment, which measures physiological responses to events in participants? daily lives, can measure fear responding to multiple trauma cues across different contexts, but it has yet to be tested in individuals with PTSD. The specific aims of this R15 proposal are 1) to test PEP and RSA as markers of conditioned fear to trauma cues using script-driven imagery, and 2) to use ambulatory physiological assessment to elucidate trauma reactivity in the daily lives of individuals with PTSD. To achieve these goals, a mixed-trauma community sample of 85 trauma-exposed participants will be recruited (43 diagnosed with PTSD). SC, PEP, and RSA reactivity to trauma scripts will be assessed with script-driven imagery in the lab. Participants will also complete three days of ambulatory physiological assessment (SC, PEP, RSA) with experience sampling of trauma cues. This project is innovative because it will examine two novel markers of fear responding to trauma cues (PEP and RSA), and because it will test ambulatory physiological assessment as a new technique to measure trauma reactivity in PTSD. The field currently lacks objective tools to monitor conditioned fear among those with PTSD, relying instead upon subjective report. This project is significant because it will provide objective markers that can be used outside the lab to monitor changes in fear responding to trauma cues, the primary target of PTSD treatment. An AREA R15 proposal, this project will train students at a minority-serving institution in innovative methods as they gain hands-on experience with significant clinical research, preparing them for careers in the health sciences.